1. Technical Field
The present invention relates to folded cascode circuits and in particular to a folded cascode circuit including feedback for improved impedance, gain, and bandwidth characteristics.
2. Description of the Related Art
Amplifier circuits may include cascode connected transistors to limit variations in collector current with collector voltage (the Early effect) and to reduce the degradation in amplifier performance at high signal frequencies (the Miller effect). Folded cascode circuits may be implemented with either field effect transistors (FETs) or bipolar transistors. Therefore, in order to keep the discussion general, the drain (collector), gate (base), and source (emitter) electrodes of a field effect (bipolar) transistor are referred to as the first, second, and third electrodes, respectively. In both cases, transistors comprising the folded cascode circuit operate in their active regions, so that the first and third electrodes of each transistor are reverse biased and forward biased, respectively, relative to the corresponding second electrode.
In a conventional cascoded amplifier circuit, the first electrode of an amplifier transistor forms a gain node which is connected to the third electrode of a second transistor. The second transistor is connected as a follower with respect to a bias voltage, and bias current provided to the gain node by a current source is divided between the amplifier transistor and the follower transistor according to a signal applied to the second electrode of the amplifier transistor. Output signals taken from the first electrode of the follower transistor accurately reflect the input signal only to the extent that the current at the first electrode of the follower transistor is determined solely by the input signal.
The follower transistor holds the voltage at the first electrode of the amplifier transistor fixed relative to the bias voltage. This reduces variations in the first electrode current of the amplifier transistor due to the Early effect and, consequently, reduces variations in the output current with the voltage at the gain node. For high frequency input signals, coupling between input signals and the gain node by the capacitance of the junction between the second and first electrodes is also reduced.
The actual improvement in the performance of cascoded amplifier circuits depends on the output impedances of the current source and amplifier transistor relative to the impedance looking into the third electrode of the follower transistor. The follower transistor is employed specifically because the third electrode of the follower transistor (source or emitter depending on the device type) provides a low impedance input for current from the gain node. However, unless the combined output impedance of the current source in parallel with the first electrode of the amplifier transistor is substantially greater than the impedance looking into the third electrode of the follower transistor, the current into the follower transistor will be a function of the voltage at the gain node. This dependence leads to limitations on the gain and bandwidth of the amplifier associated with the Early and Miller effects, respectively. The output of the current source can be cascoded to increase its output impedance. However, this approach does nothing to reduce the impedance of the third electrode of the follower transistor or improve (increase) the impedance of the amplifier transistor. In addition, it limits the compliance of the amplifier by holding the voltage of the third electrode of the follower transistor further from the reference voltage supply.